1. Field of the Invention
This invention pertains to apparatus for growing crystals, and more particularly to control systems for controlling apparatus for growing tubular crystalline bodies.
2. Prior Art
Silicon sheet used in the fabrication of semiconductor devices is frequently formed from the flat sides of tubular crystalline bodies of the type having a plurality of sides, e.g. a nonagon crystal. Apparatus of the type described in U.S. Pat. No. 4,544,528 have been used to manufacture these crystalline bodies according to the edge-defined, film-fed growth process (the EFG process). Briefly, these apparatus comprise a crucible for containing a melt of the material to be grown (e.g., silicon), a capillary die for controlling the form and shape of the grown crystal, a heater for controlling the temperature of the die and melt, a seed support assembly for supporting the seed used in growing the crystal, and a pulling mechanism coupled to the seed support assembly for drawing the tubular crystalline body out of the melt.
To produce a commercially-satisfactory silicon substrate and to ensure the crystal growth process is not prematurely terminated by either detachment or by freeze-up of the tubular crystalline body, it is essential that crystal wall thickness of the growing body be closely controlled. In capillary die apparatus of the type disclosed in U.S. Pat. No. 4,544,528, it is well known that wall thickness of the growing crystalline body varies with the size and shape of the meniscus formed between the top of the die and the bottom of the growing body. Using this relationship between wall thickness and various geometric characteristics of the meniscus, a variety of devices incorporating optical systems, such as those disclosed in U.S. Pat. Nos. 4,239,583, 4,267,151 and 4,318,769, have been developed for controlling the operation of crystal-growing apparatus. These optics-based systems comprise an optical assembly for viewing the meniscus. Using information regarding the configuration of the meniscus obtained via the optical assembly, an operator adjusts the operation of the crystal-growing apparatus so as to produce a crystalline body having the desired wall thickness.
Although satisfactory wall thickness control may be achieved with the foregoing optical control systems, certain limitations exist with respect to the use of these systems. First, an operator must continuously monitor the configuration of the meniscus and adjust the operation of the crystal growing apparatus based on what he or she observes. To avoid human error, increase crystal growth rates, and reduce the costs associated with the use of a technician, it is desirable to automate the entire crystal growth operation. Second, with known optical control systems only a relatively small portion of the entire meniscus is observable. The configuration of this small portion is not necessarily representative of the configuration of the entire meniscus.
With known crystal growing apparatus of the type illustrated in U.S. Pat. No. 4,544,528, it has been difficult and impractical to grow crystalline bodies having a length greater than about 2 meters. This difficulty is caused, in part, by the necessity to exclude air from the interior of the growing tubular crystal body by filling the latter with an inert gas such as argon, and, in part, by the fact that convection currents are generated within this volume by large temperature differences existing therein.
More specifically, an inert gas, typically argon, is injected into the interior of the growing crystalline body so as to exclude ambient air from the interior of the body. Because inert gas is heavier that ambient air, in a crystalline body having a length greater than about 2 meters, the inert gas column inside the body attains sufficient length, and hence sufficient weight, so that the column tends to sink, thereby permitting ambient air to be drawn into the body through the top end thereof.
When measures are taken to prevent ambient air from entering the crystalline body, such as by capping the top end of the body, the deleterious effects of the convection currents generated within the body increase significantly. When the tube is capped, these convection currents cause pressure fluctuations within the crystalline body which make it difficult to control the operation of the crystal growing apparatus. As a result, growth of crystalline bodies having a uniform wall thickness and lengths greater than about 2 meters is problematic.
A significant portion of the total time required to grow a crystalline body is consumed in preparing the crystal growing apparatus for the growth procedure. Consequently, to improve process efficiency it is desirable to grow crystals having lengths greater than about 2 meters.